The present invention relates to a process for treating effluents obtained from the decontamination of components of nuclear reactors, such as those resulting from the decontamination of stainless steel members which have spent a certain time in contact with the cooling fluid of a nuclear reactor, whereby said fluid can be constituted by water or liquid sodium.
It is known that in a nuclear reactor of the latter type the convection movements which occur within the sodium mass lead to the transfer of certain radioactive atoms transported by the liquid metal coolant and contaminate certain components of the reactor. One of the most frequently encountered radionuclides among those responsible for this contamination is manganese 54 formed in the reactor core by the reaction .sup.54 Fe (n, p)-.sup.54 Mn.
In order to decontaminate the stainless steel members chemical agents in solution are used which make it possible to dissolve the active products deposited in this way on the members. Various chemical agents are used, but they normally contain acids and/or bases in aqueous solution and are often completed by potassium permanganate, which acts as the oxidising agent.
Conventionally the composition of such a decontamination effluent is as follows:
acidity: 1.5 to 3 N
Na.sup.+ : 5 to 10 g/l
K.sup.+ : 0.2 to 0.5 g/l
PO.sub.4 3--: 20 to 80 g/l
SO.sub.4 2--: 5 to 10 g/l
MnO.sub.4.sup.- : 0.5 to 1 g/l
The radioactivity, mainly due to the activation products: .sup.54 Mn -.sup.60 Co -.sup.51 Cr is approximately 10.sup.-2 to 10.sup.-1 Ci/m.sup.3.
The chemical treatment of such an effluent aims on the one hand at bringing about decontamination, consisting of passing most of the radioactivity into slurry precipitates which are subsequently stored, whilst the liquid phase is discharged in the ordinary way and on the other hand obtaining a good concentration of the slurries formed during the effluent treatment, i.e. a small slurry volume compared with the initial volume of the liquid effluent to be treated. It is pointed out that in the conventional manner the decontamination quality is measured by using the decontamination factor (abbreviated to DF) which, for each radionuclide present, is expressed by the ratio between the activities of this radionuclide in the solution before and after effluent treatment. Moreover, for each of the envisaged radionuclides safety standards define in activity, i.e. in numbers of disintegrations per unit of volume and per unit of time, the maximum permissible concentration for the population (abbreviated to M.P.C.P.) in the drinking water. Finally, there can be pH problems in that the discharged water must not have too serious an influence on the biological environment due to their acidity or alkalinity.